Melt percolation and reaction in the Balmuccia peridotite.

abs. 10.1474, 01-1081.The Balmuccia lherzolite massif is traditionally considered as a document of the continental lithospheric mantle variably depleted by melting and melt extraction processes. Interaction with percolating melts were only described in the peridotite, over a maximum thickness of a few decimetres, at the contact with websteritic dikes belonging to the Cr-diopside and to the Al-Augite suites, which are the result of focused melt flow and fractionation into fractures or ductile extension zones. Recent detailed mapping of the peridotite heterogeneity, however, revealed the existence of peridotite domains which have petrochemical features ascribable to reactive porous flow of melts of various compositions. These domains can be distinguished into three types:1) regions constituted by cpx-bearing dunite or wehrlite, which contains chromitite horizons and pods up to 7 cm thick. The dunite-wehrlite bands are subconcordant to slightly discordant with the lherzolite foliation, reaching a thickness of about 6 metres and fade into zones with porphyroblastic opx. The process producing chromitite and dunite-wehrlite predated the emplacement of the oldest Cr-diopside dikes. The infiltrated melts, estimated from clinopyroxene trace element characteristics, had alkali-tholeiitic transitional affinity. 2) regions of porphyroclastic lherzolite-harzburgite, in which the porphyroclasts are represented by up to 1.5 cm large orthopyroxenes. These domains, about 10 metres large, are apparently concordant with the protogranular foliated lherzolites. The process producing the porphyroclast was broadly synchronous with the emplacement of the Cr-diopside dykes. These porphyroclastic lherzolites and harzburgites, however, contains boudinaged lenses of Cr-diopside websterite and orthopyroxenite, on their turn crossed by later Cr-diopside dykes. Texturally evident substitution of olivine by orthopyroxene indicates that the percolating melts were silica saturated.3) Regions of dunite, up to 20 metres large, containing pockets and veins constituted by large amphibole crystals, plagioclase, spinel and rutile. Dunite fades into lherzolite containing orthopyroxene porphyroblasts, suggesting the infiltration of hydrous basalt and of SiO2-saturated melts. Field relationships reveals that the process producing dunite post-dates the emplacement of all the Cr-diopside dykes and the early Al-Augite dykes.Studies on the processes, nature of the infiltrated melt and its variations during flow are in progress.